Every few years, a film comes out that is remarkably mediocre but notable because it could have been so much better. Such is the case for “RV.”

The biggest problem with this film is that the people involved are results of stellar bad decisions. The most notable bad choice is Robin Williams in the lead role of Bob Munro. He just seems uncomfortable making the movie – as if it were written not for him but for some other comedian with polar opposite talents.

The story follows a family that is out of touch with each other. They are planning a Hawaiian vacation (during which they will no doubt avoid each other), but the plans are scrapped the father (Robin Williams) is told he has to attend a business meeting in Colorado or he’s fired.

To keep with the family vacation idea, Bob rents an RV. His plan is to drive from Los Angeles to Colorado with his family, while trying to hide the fact he’s also on a business trip. What ensues is a run-of-the-mill road comedy which has been done more effectively and with funnier results by previous films. Think of “Vacation” and take out some of the funny. (Or you can just think of “Vegas Vacation,” and you’ll be pretty close to the mark.)

After the screening of this film, I was speaking with a friend, and he suggested that Steve Martin would have been better in the role. That’s when everything came together. It was like a religious epiphany. Steve Martin would have been perfect for this role. In fact, if he had been in it, it might have actually been funny. The movie just seemed like your standard modern Steve Martin family comedy.

But not Robin Williams.

Director Barry Sonnefeld is equally out of place at the helm of this movie. I generally like Sonnefeld’s work, and I greatly appreciate his snappy visual style we’ve all seen in movies like “Men in Black” and the sadly forgotten “Big Trouble.” However, there’s no punch to the directing style of this film. It’s a huge step down from Sonnefeld’s better works to the mundane “RV.”

Then there’s the writing. So much of this film’s script seems to have stemmed from improvisation and goofing off on set. That’s not a new thing for a Robin Williams movie, and usually it results in some great comedy. However, here – like much of the elements of the movie – these moments are horrendously out of place.

For example, the filmmakers seem to get quite a kick out of Cheryl Hines being able to sing artificially low for her normally squeaky voice. Sure, it’s funny when it happens once, but three minutes of her singing Ronny and the Daytonas’ “GTO” gets wearing fast. And they don’t let up because you’re treated to a similar rendition of “Route 66” with her false baritone at the end of the film.

When it comes to Robin Williams, his improv-style comedy is funny, but painfully out of place. The character of Bob Munro is supposed to be a corporate cog who is getting increasingly out of touch with his family. He’s not particularly a witty guy, yet somehow he manages to provide these strings of goofy humor. It just doesn’t fit. Again, it’s painfully apparent that Williams’ appearance in this film is the biggest casting faux pas since Julia Roberts as Tinkerbell, Tommy Lee Jones as Two-Face and Kevin Costner in almost anything he does.

The only hope for this film is the generic family crowd. The people in the theatre where I saw it seemed to like it. There’s nothing too risqué in the film, and there’s enough low-brow slapstick comedy. And in case that doesn’t sell you on it, there’s poop humor. What RV road comedy wouldn’t be complete without a fecal storm from the overloaded septic tank?